It is narrated from Sayyiduna Ali (may Allah be pleased with him): There is no marriage in the absence of a guardian and witnesses. In the narration of (Yazid bin) Harun it reads: There is no marriage without the permission of a guardian and without the presence of two upright witnesses.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 13645
Hadith Gradingمحدثین:حسن لغيره دون قول (وَلاَ نِكَاحَ إِلاَّ بِشُهُودٍ)
Hadith Takhrij«حسن لغيره دون قول (وَلاَ نِكَاحَ إِلاَّ بِشُهُودٍ)»