Mishkat al-Masabih 7 Fasting كتاب الصوم 88

وَعَن أَبِي قَتَادَةَ قَالَ: سُئِلَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ عَنْ صَوْمِ الِاثْنَيْنِ فَقَالَ: «فِيهِ وُلِدْتُ وَفِيهِ أُنْزِلَ عَلَيَّ» . رَوَاهُ مُسْلِمٌ

He said that when God’s messenger was asked about fasting on Monday he said, “On it I was born and on it the revelation was first sent down to me.”* *The meaning seems to be that as this day is so distinguished, one could not choose a better one. Muslim transmitted it.