Musannaf Ibn Abi Shayba 3:6026

Abu Bakr narrated to us, he said: Abu Khalid al-Ahmar narrated to us, from Sa'd ibn Ishaq, from 'Asim ibn 'Umar ibn Qatadah, from Mahmud ibn Labid, he said: The Messenger of Allah (may Allah bless him and grant him peace) said: "Beware of the shirk (polytheism) of the secrets." They said: "And what is the shirk of the secrets?" He said: "That one of you stands and adorns his prayer, striving [to do so], so that people may look at him; that is the shirk of the secrets."