Rule and benefits of joining feet's together during solat

Assalamualaikum warahmatullahi wabarakatuhu Please if anyone can help me with this question I will appreciate it What are the rules I mean is it compulsory to join feet's during prayer When I was young I use to hear that shaytan use to pass through the gap between you and your partner in solat if you don't join your feet and make you prayer invalid is it true Please I need more evidence please Jazakumul llahu khayran