Musannaf Ibn Abi Shayba 33:399

Abdah ibn Sulayman narrated to us, from Muhammad ibn Amr, from Yahya ibn Abd al-Rahman ibn Hatib, who said: Abu Waqid al-Laythi said: We examined the deeds to see which of them is best, and we did not find anything more helpful for seeking the Hereafter than asceticism (zuhd) in the world.