Musnad Ahmad 962:55

Ali ibn Ishaq narrated to us, Abdullah ibn al-Mubarak informed us, Yahya ibn Ayyub informed us, from Ubayd Allah ibn Zahr, from Ali ibn Yazid, from al-Qasim, from Abu Umamah, from the Prophet, peace and blessings of Allah be upon him, who said: Allah said: "The most beloved of what My servant worships Me with, to Me, is sincerity (al-nush) to Me."