Musnad Ahmad 26:219

Husayn ibn Ali narrated to us, from Za'idah, from Sulayman, from Shaqiq, from Abdullah, who said: "I prayed with the Messenger of Allah, peace and blessings of Allah be upon him, and he prolonged the standing, until I contemplated a bad thing." He said: We said: "And what was it?" He said: "I contemplated sitting down."