Is there any valid proof for Salaat al-Tasaabeeh?  

There are ahaadeeth concerning Salaat al-Tasaabeeh which are attributed to the Prophet (peace and blessings of Allaah be upon him), and which were classed as hasan by some of the scholars, but many scholars said that they are da’eef (weak) and that this prayer is not prescribed in Islam. 

The Standing Committee was asked about Salaat al-Tasaabeeh, and they answered: 

Salaat al-Tasaabeeh is a bid’ah (innovation) and the hadeeth concerning it is not proven, rather it is munkar. Some of the scholars mentioned it among the mawdoo’ (fabricated) ahaadeeth. 

See Fataawa al-Lajnah al-Daa’imah, vol. 8, p. 163 

Shaykh Ibn ‘Uthaymeen said: Salaat al-Tasaabeeh is not prescribed, because the hadeeth is da’eef (weak). Imaam Ahmad said there is no sound evidence for it. Shaykh al-Islam Ibn Taymiyah said it is a fabrication, and he said that none of the imaams recommended it. He (may Allaah have mercy on him) spoke the truth, because whoever examines this prayer will find that it is odd in the way it is performed … Moreover, if it were prescribed, it would have been something that was reported in many reports because of its great virtue and reward. Since there are no such reports and none of the imaams described it as being recommended, we know that it is not proven. The oddness in the way in which it is performed is as described in the hadeeth narrated concerning it, which says that it is to be performed once every day, or once every week, or once every month, or once every year, or once in a lifetime. This indicates that these reports are not sound; if it were prescribed, it would be something to be done on a regular basis, and one would not be given such a variety of options. 

On this basis, people should not do this prayer. And Allaah knows best. 

Fataawa Manaar al-Islam, 1/203