Nafi’ (may Allah have mercy on him) says that whenever Sayyiduna Ibn Umar (may Allah be pleased with them both) was asked about Witr, he would say: If I wish to perform Witr before sleeping, and then later intend to pray during the night, I add one rak’ah to the rak’ah I already prayed for Witr, then I continue to pray two rak’ahs at a time. When I have completed my prayer, I combine one rak’ah with two rak’ahs and perform Witr, because the Prophet (peace and blessings be upon him) commanded that the last prayer of the night should be Witr.