It is narrated from Hazrat Masruq that he presented himself before Sayyida Aisha (may Allah be pleased with her) and asked her about the prayer of the Messenger of Allah (peace and blessings be upon him). She replied that the Prophet (peace and blessings be upon him) used to pray thirteen rak‘ahs at night, then he started praying eleven rak‘ahs and left two rak‘ahs, and when the Prophet (peace and blessings be upon him) passed away, at that time he used to pray nine rak‘ahs at night. The last prayer of the Prophet (peace and blessings be upon him) at night was Witr. Sometimes the Prophet (peace and blessings be upon him) would come to his bed (and rest), then Sayyiduna Bilal (may Allah be pleased with him) would come and inform the Prophet (peace and blessings be upon him) about the prayer (so the Prophet (peace and blessings be upon him) would go to lead the prayer). Imam Abu Bakr (may Allah have mercy on him) says that we act according to those narrations which we have mentioned in Kitab al-Kabir regarding the number of rak‘ahs of the Tahajjud prayer of the Prophet (peace and blessings be upon him). The difference in the number of rak‘ahs among the narrators is similar to the difference found in those narrations which I have mentioned in this book. The Prophet (peace and blessings be upon him) used to perform more rak‘ahs on some nights than on others. Thus, whatever number of rak‘ahs or manner of the Tahajjud prayer has been reported by all the noble Companions (may Allah be pleased with them), the pure wives of the Prophet (may Allah be pleased with them all), or other women, the Prophet (peace and blessings be upon him) performed the prayer in that number and manner on some nights. And this difference is of the permissible kind. Therefore, it is permissible for the one who prays to perform the prayer in any number or manner reported from the Prophet (peace and blessings be upon him) that he prefers. There is nothing prohibited in this for anyone.
And the evidence that the Prophet, peace and blessings be upon him, used to pray thirteen rak‘ahs at night as Ibn Abbas reported, then he reduced two rak‘ahs and used to pray eleven rak‘ahs at night as Abu Salamah reported from Aisha, then he reduced two rak‘ahs from the night prayer and used to pray nine rak‘ahs at night as Abdullah ibn Shaqiq reported from Aisha.
Hadith Referenceصحيح ابن خزيمه / جماع أبواب صلاة التطوع بالليل / 1168