Hadith 21, 40 Hadith an-Nawawi
On the authority of Abu `Amr — and he is also called Abu `Amrah — Sufyan bin Abdullah ath- Thaqafee (may Allah be pleased with him) who said:
I said, "O Messenger of Allah, tell me something about al-Islam which I can ask of no one but you." He (peace and blessings of Allah be upon him) said, "Say I believe in Allah — and then be steadfast." [Muslim]
عَنْ أَبِي عَمْرٍو وَقِيلَ: أَبِي عَمْرَةَ سُفْيَانَ بْنِ عَبْدِ اللَّهِ رَضِيَ اللهُ عَنْهُ قَالَ: "قُلْت: يَا رَسُولَ اللَّهِ! قُلْ لِي فِي الْإِسْلَامِ قَوْلًا لَا أَسْأَلُ عَنْهُ أَحَدًا غَيْرَك؛ قَالَ: قُلْ: آمَنْت بِاَللَّهِ ثُمَّ اسْتَقِمْ" . 
[رَوَاهُ مُسْلِمٌ].