Musnad Ahmed: Hadith 4270

al-Waleed bin 'Uqbah delayed the prayer on one occasion. ‘Abdullah bin Mas'ood stood up and gave the iqamah (call immediately preceding the prayer), then he led the people in prayer. Al-Waleed sent word to him asking: What made you do what you did? Did instructions come to you from Ameer al-Mu'mineen with regard to what you did or have you introduced something? He said: No instructions came to me from Ameer al-Mu'mineen and I did not introduce anything, rather Allah, may He be glorified and exalted, and His Messenger insisted that we should not wait for you concerning prayer when you are busy.