Here are some mentions of where Müller examined his heart relative to his motives in prayer.
"So far as I remember, I brought even the most minute circumstances concerning the Orphan House before tha Lord in my petitions, being conscious of my own weakness and ignorance. There was, however, one point I never had prayed about, namely, that the Lord would send children; for I naturally took it for granted that there would be plenty of applications. The appointed time came, and not even one application was made. This circumstance now led me to lie low before my God in prayer, and to examine my heart once more as to all the motives concerning it; and being able, as formerly, to say that his glory was my chief aim, i. e., that it might be seen that it is not a vain thing to trust in the living God, and still continuing in prayer. I was at last brought to this state, that I could say from my heart that I should rejoice in God being glorified in this matter, though it were by bringing the whole to nothing. But as still, after all, it seemed to me more tending to the glory of God to establish and prosper the Orphan House, I could then ask him heartily to send applications. I enjoyed now a peaceful state of heart concerning the subject, and was also more assured than ever that God would establish it. The very next day the first application was made, and within a short time forty-three applied."—From his Narrative (1877), p. 137.