The grave is the first stage of the Hereafter

Hani' (the freed slave of Uthman) narrates that whenever Sayyiduna Uthman bin Affan (may Allah be pleased with him) stood at a grave, he would weep so much that his blessed beard became wet (with tears). It was said to him: You mention Paradise and Hell and do not weep this much, yet you weep this much on seeing a grave? He replied that the Messenger of Allah (peace and blessings be upon him) said: "Indeed, the grave is the first of the stages of the Hereafter. If a person is saved from it, the stages after it will be easier than it; and if he is not saved from it, the stages after it will be harder than it." And the Messenger of Allah (peace and blessings be upon him) also said: "I have not seen any sight except that I found the sight of the grave more dreadful than it."

Sayyiduna Ya'la bin Murrah (may Allah be pleased with him) narrates: I travelled with the Prophet (peace and blessings be upon him) many times, and I saw that whenever he (peace and blessings be upon him) passed by the corpse of a human being, he would order it to be buried; he (peace and blessings be upon him) did not ask whether it was a Muslim or a disbeliever.
This hadith is sahih (authentic) on the condition of Muslim, but the two Shaykhs (Bukhari and Muslim) did not record it.