The Prophet (peace and blessings be upon him) said:
"A man's sending a messenger to someone (to announce his coming) is itself his asking for permission."
Hadith Referenceصحیح الجامع الصغیر / حرف الراء / 3504
Hadith Gradingالألبانی:صحيح
Hadith Takhrij[د] عن أبي هريرة. الإرواء ١٩٥٥: خد، حب.