Nafi reported,

I was with Abdullah bin Umar in Makkah on a night when the sky was overcast. Fearing that dawn was near, Abdullah prayed a single rakah as his witr. Then the clouds cleared and he saw that there was still night left, so he added another single rakah to make it an even number. After that he prayed two rakahs at a time, and when he feared dawn was approaching, he prayed a single rakah as his witr.

و حدثني عن مالك

عن نافع أنه قال

كنت مع عبد الله بن عمر بمكة والسماء مغيمة فخشي عبد الله الصبح فأوتر بواحدة ثم انكشف الغيم فرأى أن عليه ليلا فشفع بواحدة ثم صلى بعد ذلك ركعتين ركعتين فلما خشي الصبح أوتر بواحدة