Assalaamu alaykum. I have read the following: Amr ibn Al-Aswad used to have an expensive garment of 200 dirhams that he wore exclusively for the night prayer. (Narrated by Ibn Kathir in Al-Bidayah wan-Nihayah) Is this permissible? I mean, for a man to buy a a special piece of clothing (like a Thawb or Jalabah) and only use it for night prayers. Or is that an innovation (Bid'ah)? Are there any reports concerning this from the Salaf (pious predecessors)?

All perfect praise be to Allaah, The Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ʻalayhi wa sallam, is His slave and Messenger. 

In principle, there is no harm in wearing a nice garment exclusively for the night prayer because the Muslim is enjoined to wear good clothes for prayer. Allaah, The Exalted, says (what means): {O children of Adam, take your adornment at every masjid (prayer)...} [Quran 7:31] Many of the righteous predecessors used to wear their best clothes for prayer. Al-Haafith Ibn Rajab wrote:

“Indeed, it is not disliked to wear a fine garment for prayer unless it is feared to distract the praying person or it is worn out of arrogance. It was narrated that Tameem Ad-Daari used to wear a garment that he bought for one thousand dirhams for the night prayer. The Prophet, sallallaahu ʻalayhi wa sallam, sometimes wore Yemeni garments (woven with red and black stripes) and fancy cloaks, and it has not been reported that he avoided performing the prayer in them. He, sallallaahu ʻalayhi wa sallam, returned the garment bearing designs because it distracted him during the prayer. Allaah, The Exalted, says (what means): {O children of Aadam (Adam), take your adornment at every masjid (prayer)...} [Quran 7:31] We have previously cited the statement of Ibn ʻUmar which reads, 'Allaah is the worthiest of those for whom one should adorn himself.' Abu Daawood cited a Mursal report (i.e. a hadeeth narrated by a Tabi'i [successor of the Prophet's Companions]) on the authority of ʻUbaydullaah ibn ʻAbdullaah ibn ʻUtbah that says that the Prophet, sallallaahu ʻalayhi wa sallam, loved the clean apparel and a good smell for prayer. The righteous predecessors used to wear their best clothes for prayer and did not consider it an indication of arrogance...”

Knowing that the honorable Companion Tameem Ad-Daari wore a special garment for the night prayer shows that it is not a Bid'ah.

Allaah knows best.