What indicates that his command to treat them equally in gifts is a recommendation, not an obligation

Sayyiduna Nu'man bin Bashir (may Allah be pleased with them both) says: My father brought me to the Messenger of Allah (peace and blessings be upon him) and said: O Messenger of Allah, bear witness that I have given Nu'man such-and-such from my wealth. He (peace and blessings be upon him) said: "Have you given all your sons the same as you have given Nu'man?" He said: No. He (peace and blessings be upon him) said: "Make someone other than me a witness. Would it not please you that they all be equal towards you in kindness?" He said: Of course. He (peace and blessings be upon him) said: "Then, no."
In the wording of al-Sha'bi (may Allah have mercy on him): "Would it not please you that they be equal towards you in kindness and gentleness?" He said: Yes. He (peace and blessings be upon him) said: "Then make someone other than me a witness to this."

It is narrated from Sayyiduna Nu'man bin Bashir (may Allah be pleased with them both): My father gave me a gift. Isma'il bin Salim, among the narrators, said: He gave him a slave. Nu'man (may Allah be pleased with him) said: My mother, Amrah bint Rawahah (may Allah be pleased with her), said: Go to the Messenger of Allah (peace and blessings be upon him) and make him a witness. So he came to the Prophet (peace and blessings be upon him) and mentioned it to him, saying: I have given my son Nu'man a gift, and Amrah has asked me to make you a witness to it. He (peace and blessings be upon him) asked: "Do you have other children besides him?" He said: Yes. He (peace and blessings be upon him) said: "Have you given all of them the same as Nu'man?" He said: No. Some of the hadith narrators reported that he (peace and blessings be upon him) said: "This is injustice." And some said: It is a gift singled out for him to the exclusion of the others, or a sham gift under pressure. He (peace and blessings be upon him) said: "Then make someone other than me a witness." In the wording of Mughirah: "Would it not please you that they be equal towards you in kindness and gentleness?" He said: Yes. He (peace and blessings be upon him) said: "Make someone other than me a witness." And Mujalid mentioned in his narration: "They have a right over you that you be just between them, just as you have a right over them that they treat you with kindness."

Al-Sha'bi (may Allah have mercy on him) says that he heard the long story from Nu'man bin Bashir (may Allah be pleased with him), and at its end he said that the Prophet (peace and blessings be upon him) said: "I will not bear witness to this; this is injustice. Make someone else a witness to this. Be just between your children in gifts, just as you love that they be just towards you in kindness."

It is narrated from Sayyiduna Nu'man bin Bashir (may Allah be pleased with them both): My father gave me a gift, then he brought me to the Prophet (peace and blessings be upon him) to make him a witness. He (peace and blessings be upon him) asked: "Have you given this to all your sons?" He said: No. He (peace and blessings be upon him) said: "Do you not want from them the same kindness that you want from this one?" He said: Of course. He (peace and blessings be upon him) said: "Then I will not bear witness." Ibn Awn said: I narrated this to Muhammad, meaning Ibn Sirin, and he said: We were told that he (peace and blessings be upon him) said: "Keep your children close to equal."
Imam al-Shafi'i (may Allah have mercy on him) says: Sayyiduna Abu Bakr (may Allah be pleased with him) favoured Sayyidah Aishah (may Allah be pleased with her) with a gift.
The Shaykh says: Urwah bin al-Zubayr says: Sayyidah Aishah (may Allah be pleased with her) said: Sayyiduna Abu Bakr (may Allah be pleased with him) had given me, from his wealth, dates to be harvested amounting to twenty wasqs. When death approached him, he sat up with his knees drawn up, recited the testimony of faith, and then said: To proceed: my dear daughter, of all people the one I would most love to be well-off after me is you. I had given you dates to be harvested amounting to twenty wasqs from my wealth. By Allah, I wish you had taken possession of it and harvested it, but today it is the wealth of the heirs, and they are your two brothers and two sisters. Sayyidah Aishah (may Allah be pleased with her) said: O my father, this is Asma; so who is the other? He said: The one in the womb of the daughter of Kharijah; I think it is a girl. Sayyidah Aishah (may Allah be pleased with her) said: Even if you had given me more than this, I would have returned it to you.
Imam al-Shafi'i (may Allah have mercy on him) says: Sayyiduna Umar (may Allah be pleased with him) favoured Asim bin Umar (may Allah be pleased with him) by giving him something, and Sayyiduna Abd al-Rahman bin Awf (may Allah be pleased with him) favoured the children of Umm Kulthum.

It is reported from Nafi' that Sayyiduna Ibn Umar (may Allah be pleased with them both) allotted three or four head (of slaves or livestock) to some of his children to the exclusion of others.

Qasim bin Abd al-Rahman al-Ansari (may Allah have mercy on him) and Sayyiduna Ibn Umar (may Allah be pleased with them both) went to a man of the Ansar. They bargained with him over land of his, and he bought it from him. Then a man came and said: "I saw that you bought land and gave it in charity." Sayyiduna Ibn Umar (may Allah be pleased with them both) said: "This land belongs to my son Waqid, for he is poor." He gave it to him as a gift to the exclusion of his other children.

Abd al-Rahman bin al-Qasim narrated that his father used to make allotments to some of his children to the exclusion of others.
It is reported from Umar bin al-Munkadir (may Allah have mercy on him) that the Messenger of Allah (peace and blessings be upon him) said: "Every owner of wealth has more right to his wealth." Ibn Wahb (may Allah have mercy on him) said: He may do with it whatever he wishes.