Sayyiduna Imam al-Husayn bin Ali (may Allah be pleased with them both) narrates: On the night of al-Aqabah, the Ansar came to the Prophet (peace and blessings be upon him) to give the pledge of allegiance. The Prophet (peace and blessings be upon him) said: "O Ali! Get up. O Ali! Take the pledge from them!" He said: "O Messenger of Allah! On what shall I take the pledge from them?" The Prophet (peace and blessings be upon him) said: "On this: that Allah Almighty will be obeyed and not disobeyed, and that you will protect the Messenger of Allah, his family (Ahl al-Bayt) and his offspring just as you protect yourselves and your children."
Al-Tabarani recorded it in al-Mu'jam al-Awsat through a narrator named Abdullah bin Marwan, who is da'if (weak), though he has also been declared trustworthy.