Basil of Caesarea, on Ps 61:9

Then he brings up a decision for all that was said, not now from his own words but one that he heard from God. "God has spoken once, these two things have I heard," he says. And let it not disturb anyone that what was said is, as it were, incredible, namely, that God spoke once and the prophet heard two things. For it is possible for someone to speak once but for the things spoken on the one occasion to be many. In fact, when a certain person met someone once, he discussed many things. The one who heard his words is able to say, "He talked with me once, but he spoke about many things." This is what was meant on the present occasion, the manifestation of God occurred to me once, but there are two matters about which he talked. He did not say, "God spoke of one thing, but I heard these two." If he had, the statement would seem to have some discrepancy in it. What were the two things that he heard? "That power belongs to God, and mercy to you, O Lord." God is powerful, he says, in judgment, and likewise merciful. Therefore do not trust in iniquity, and do not hand yourself over to riches. Do not choose vanity; do not carry around the corrupt lawcourt of your soul. Knowing that our Lord is mighty, fear his strength and do not despair of his kindness. Now, in order that we may not do wrong, fear is good; and in order that he who has once slipped into sin may not throw himself away through despair, the hope of mercy is good. For power belongs to God, and mercy is from him.