It is narrated from Sayyiduna Abu Hurairah (may Allah be pleased with him) that the Prophet (peace and blessings of Allah be upon him) said: "When a person sends someone as a messenger to call another, that itself is permission."
Hadith Referenceالادب المفرد / كتاب الاستئذان / 1076
Hadith Gradingالألبانی:صحيح
Hadith Takhrij«صحيح : أخرجه أبوداؤد ، كتاب الأدب : 5189 - أنظر الإرواء : 1955»