Umm Salamah (may Allah be pleased with her) reported that when the Messenger of Allah (may Allah's peace and blessings be upon him) married her, he stayed three nights with her and said: "Indeed, you are not inferior in the sight of your family. If you wish, I can stay seven (nights) with you, but if I stay seven with you, I will also stay seven with my (other) wives."
Umm Salamah (may Allah be pleased with her) mentioned that when the Prophet (may Allah's peace and blessings be upon him) married her, he gave her two options to choose from. He would either stay seven nights with her and then stay the same number of nights with each of his other wives, or he would stay only three nights with her and then give each of his wives their normal nightly turns. Before giving her the choice, the Prophet preambled his excuse for the three-night option by informing Umm Salamah that she was dear to him, and that in no way was she inferior in his sight; so the three-night option was not because she had any deficiency, but because it was the right thing to do.