Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) said: In the time of the Messenger, a man was killed and the killer was not known. The Prophet (peace and blessings be upon him) said: "A killing has taken place while I am among you, and the killer is not known. If the people of the heavens and the people of the earth together were to kill a believer, Allah would certainly punish them; if He does not wish to (punish them), that is up to Him."
And in the hadith of Abu Abdullah, the words are: If the people of the heavens and the people of the earth were to gather to kill a believer, Allah would certainly punish them.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجنايات / 15864