Muwatta Imam Malik (Riwayat Yahya)

It is narrated from Hazrat Nafi that I was with Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) on the way to Makkah and there were clouds covering the sky, so Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) feared that morning had come. Thus, he performed one rak'ah of witr, then the clouds cleared and he saw that the night was still remaining, so he performed another rak'ah and made it two rak'ahs. After that, he performed two rak'ahs at a time, and then when he feared that morning had come, he performed one rak'ah of witr.