Mishkat al-Masabih 1 Faith كتاب الإيمان 14

وَعَن سُفْيَان بن عبد الله الثَّقَفِيّ قَالَ: قُلْتُ: يَا رَسُولَ اللَّهِ قُلْ لِي فِي الْإِسْلَامِ قَوْلًا لَا أَسْأَلُ عَنْهُ أَحَدًا بَعْدَكَ وَفِي رِوَايَةٍ: غَيْرَكَ قَالَ: ” قُلْ: آمَنْتُ بِاللَّه ثمَّ اسْتَقِم. رَوَاهُ مُسلم

Sufyan b. ‘Abdallah ath-Thaqafi reported that he said, “Messenger of God, say something about Islam concerning which I shall need to ask no one after you are gone.” (A version has “anyone else”.) He said, “Say, ‘I believe in God’, then keep to the straight path.” Muslim transmitted it.