In the book of Zadul Ma’aad, the late Allamah Majlisi (RA) wrote:

It is well known that the reason for stressing on Ziarat Arbaeen is that Imam Sajjad (AS) entered Karbala on this day after returning from Shaam and on this day he joined the heads of martyrs with their bodies. It is also narrated that Imam Sadeq (AS) taught Safwan Ibn Mehran about Ziarat Arbaeen and this instruction is a sign of the importance and commemoration of Ziarat Arbaeen [1]. In another narration, Imam Askari (AS) said: A believer must have five signs: To perform 51 Rok’ats of Salaat (per day; Wajib and Mostahab), reciting  Ziarat Arbaeen, wearing ring on the right hand, prostrate on the soil, and reciting Bismillahe Rahmane Rahim louder when performing Salaat [2].

[1] Beharul Anwaar, Vol. 9, Page 331.

[2] Misbahul Mojtahid, Sheikh Toosi, Page 787.

Source: en.shafaqna.com