Hadith: Say "I believe in Allah" and then be upright

«عن أبي عمرو - وقيل أبي عمرة - سفيان ابن عبد الله رضي الله عنه قال: قلت يا رسول الله قل لي في الإسلام قولاً لا أسال عنه أحداً غيرك، قال قل آمنت بالله، ثم استقم»
[صحيح] - [رواه مسلم]

Abu ‘Amr – or Abu ‘Amrah – Sufyān ibn ‘Abdullah (may Allah be pleased with him) reported that he said: "O Messenger of Allah, tell me something about Islam regarding which I can ask none but you." He (may Allah’s peace and blessings be upon him) said: "Say 'I believe in Allah' and then be upright."
Sahih/Authentic. - [Muslim]

Explanation

Sufyān ath-Thaqafi said to the Messenger of Allah (may Allah’s peace and blessings be upon him): O Messenger of Allah, tell me something in Islam about which I will not ask anyone after you. In other words, a self-evident statement that does not need explanation or clarification from anyone else. A brief statement that comprises all the goodness of this life and the Hereafter and leads to triumph and success - that I may memorize and act upon. This shows how the Companions (may Allah be pleased with them) were keen on doing good and acquire good understanding of their religion. In response, the Prophet (may Allah’s peace and blessings be upon him) said: "Say 'I believe in Allah' and then be upright", observing His commands and prohibitions, as the verse says: {Indeed, those who say, “Our Lord is Allah”, then remain upright, they will have no fear, nor will they grieve} [Surat al-Ahqāf: 13]. His words "I believe in Allah" do not mean mere utterance of this phrase, but uttering it by the tongue, and believing in it within the heart, and acting upon it through body organs. Indeed, faith consists of belief by the heart, acknowledgment by the tongue, and actions by the body organs. It is composed of these three elements. Belief is mentioned here in general terms, thus including all inward and outward matters. This means that a person believes in Allah Almighty and believes in the content of His Book and the Sunnah of His Prophet (may Allah’s peace and blessings be upon him) and thereafter steadfastly adheres to the truth and guidance, which is belief in Allah that comprises all inward and outward matters. Uprightness lies in between two extremes. It is straightforwardness that is free from crookedness. "Be upright" means: Adhere to this upright religion and straightforward belief, and do not swerve to the right or left, until you reach the destination.