Muwatta Malik, Book 7, Hadith 19 (USC-MSA) ⓘ

Yahya related to me from Malik that Nafi said, "I was with Abdullah ibn Umar in Makka. The sky was clouded over and Abdullah feared that dawn was approaching so he prayed one raka for witr. Then the clouds cleared and he saw that it was still night, so he made his prayers even with one raka. Then he continued to pray two rakas at a time, until when he feared the approach of dawn, he prayed one raka for witr."

وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ نَافِعٍ، أَنَّهُ قَالَ كُنْتُ مَعَ عَبْدِ اللَّهِ بْنِ عُمَرَ بِمَكَّةَ وَالسَّمَاءُ مُغِيمَةٌ فَخَشِيَ عَبْدُ اللَّهِ الصُّبْحَ فَأَوْتَرَ بِوَاحِدَةٍ ثُمَّ انْكَشَفَ الْغَيْمُ فَرَأَى أَنَّ عَلَيْهِ لَيْلاً فَشَفَعَ بِوَاحِدَةٍ ثُمَّ صَلَّى بَعْدَ ذَلِكَ رَكْعَتَيْنِ رَكْعَتَيْنِ فَلَمَّا خَشِيَ الصُّبْحَ أَوْتَرَ بِوَاحِدَةٍ ‏.‏