And it is also permissible that a voluntary act (nafl) be abrogated and made obligatory again, just as in the beginning it was obligatory.

Lady Aisha (may Allah be pleased with her) narrates that the Messenger of Allah (peace and blessings be upon him) went from home to the mosque at midnight and performed (Tahajjud) prayer. Some people also prayed with him. In the morning, people informed each other about this. Then, when the third night came, many people gathered in the mosque. The Prophet (peace and blessings be upon him) came and they performed prayer with him. Then, when the fourth night came, the mosque could not accommodate all the people, and the Messenger of Allah (peace and blessings be upon him) did not come to them. Some of them kept calling out, "Prayer, prayer," to the Prophet (peace and blessings be upon him), but the Messenger of Allah (peace and blessings be upon him) did not come out. This continued until he came out for the Fajr prayer. Then, when the Prophet (peace and blessings be upon him) completed the Fajr prayer, he stood up, turned his blessed face towards the people, delivered a sermon, praised and glorified Allah Almighty, and then said: "As for what follows, indeed your situation (attendance) was not hidden from me, but I feared that the night prayer might be made obligatory upon you, and then you would be unable to perform it." These are the words of the narration of Al-Durqutni.