Is group Dhikr after the prayer allowed, and what is the ruling on saying "Tahleel"(Saying Lā ilāha illallāh, waḥdahu lā sharīka lahu, lahul-mulku wa lahul-ḥamdu, yuḥyī wa yumītu, wa huwa `alā kulli shay’in qadīr) ten times after the prayer, and is there a valid evidence for it?