Musannaf Ibn Abi Shayba 1:89

Ibn ‘Ulayyah narrated to us, from Muhammad ibn Ishaq, from Shaybah ibn Nasah, who said: I accompanied al-Qasim ibn Muhammad to Makkah. I saw him, when he performed ablution for prayer, inserting the fingers of his hands between the toes of his feet. He said: And he was pouring water over them. I said to him, "O Abu Muhammad, why do you do this?" He said, "I saw ‘Abdullah ibn ‘Umar doing it."