The Battle of Tabuk

It is narrated from Sayyiduna Umar bin al-Khattab (may Allah be pleased with him) that we set out with the Messenger of Allah (peace and blessings be upon him) (on a journey), until, when we reached the place called Ayn al-Rum, and this was the expedition of Tabuk, we were struck by severe hunger. I said: "O Messenger of Allah! Tomorrow we will meet the enemy; they are well fed and full, while we are hungry (so how will we fight?)." At this the Prophet (peace and blessings be upon him) addressed the people and then said: "Whoever has surplus food, let him bring it to us." Then he (peace and blessings be upon him) spread out a leather mat, and a little over twenty sa' of grain were brought and placed on it. Then the Messenger of Allah (peace and blessings be upon him) sat down and supplicated for blessing, then he called the people and said: "Take (from it)." The people began to take, until things reached the point where a man would tie up the sleeve of his shirt (at the bottom) and fill it with grain, and even so some food was left over. Then the Messenger of Allah (peace and blessings be upon him) said: "I bear witness that there is no true god but Allah and that I am the Messenger of Allah. Whoever says these words sincerely and truthfully will never enter the Fire."
The chain of this hadith contains Yazid bin Abi Ziyad and Asim bin Ubaydullah, who are da'if (weak) narrators. However, other supporting evidence for the miracle of the multiplication of food and the virtue of the testimony of faith exists in sahih hadith.

It is narrated from Sayyiduna Abu Qatadah (may Allah be pleased with him) that when we were returning from the expedition of Tabuk (then he narrated the whole hadith);
and in this narration it is also mentioned that he (peace and blessings be upon him) said: "Whoever among you meets any of those who stayed behind (from the expedition without excuse), let him neither speak to them nor sit in their company."
And in it these words are also reported, that he (peace and blessings be upon him) said: "This is Taybah (Madinah). My Lord has settled me here. This city expels the filth and impurity of its inhabitants just as the blacksmith's bellows removes the rust and dross of iron. So whoever among you meets any of those who stayed behind, let him neither speak to them nor sit with them."

It is narrated from Sayyiduna al-Mughirah bin Shu'bah (may Allah be pleased with him) that he said: We were with the Messenger of Allah (peace and blessings be upon him) on a journey. While we were travelling with him (peace and blessings be upon him) at night, the she-camel of the Messenger of Allah (peace and blessings be upon him) suddenly turned aside, so I followed him (peace and blessings be upon him). When he (peace and blessings be upon him) saw me, he asked: "Where have the people got to?" I said: I left them at such and such a place. Then the Messenger of Allah (peace and blessings be upon him) made his she-camel kneel and dismounted from it. Then he (peace and blessings be upon him) went off until he was hidden from my sight, and he stayed away for as long as it takes a man to relieve himself. Then the narrator mentioned the whole hadith about wiping over the leather socks, and at the end of it he related that he (peace and blessings be upon him) then asked me: "Do you need anything?" I said: I have no need. So we mounted and rode until we caught up with the people.
Its chain is sahih (authentic).

It is narrated from Bakr bin Abdullah al-Muzani (may Allah have mercy on him) that the Messenger of Allah (peace and blessings be upon him) said: "Who will take this letter of mine to Caesar (of Rome), and in return Paradise (is promised) for him?" A man said: (Will this reward be given) even if he is not killed? He (peace and blessings be upon him) said: "Yes, even if he is not killed." So that man set off, reached Caesar and presented the letter to him. He read the letter and said: Go back to your Prophet and tell him that I am his follower (a Muslim), but I do not wish to give up my kingdom. And he sent some dinars with the messenger to the Messenger of Allah (peace and blessings be upon him). The man returned and informed him (peace and blessings be upon him) of what had happened, and the Messenger of Allah (peace and blessings be upon him) said: "He has lied (in claiming to be my follower)." And he (peace and blessings be upon him) distributed those dinars.
This report is in Musnad al-Harith (4316). Bakr bin Abdullah al-Muzani is a Successor (Tabi'i), so this hadith is mursal (with a missing link).