His statement, “then [he should pray] facing a line,” means: he should pray facing a line. The line must leave a mark on the ground, because in former times the ground was covered with sand or pebbles, and when a person drew a line, it produced a clear mark. But mosque floors nowadays are carpeted. So can we say that a line formed by the colouring of the carpet suffices in place of a line that leaves a mark?

Some scholars have said: Anything that one considers to be a sutrah (prayer barrier) suffices. The apparent implication of this is that even a coloured line would suffice, but there is some reservation about this.

The apparent view is that these coloured lines are not sufficient. However, if there happened to be a raised thread at the edge of a mat or carpet, it would be valid for it to serve as a sutrah, because it is raised.

The evidence for this is that the Prophet ﷺ said: “…and whoever cannot find one, let him draw a line.” (2) Al-Hafiz Ibn Hajar said concerning this hadith in Bulugh al-Maram: “Those who claimed it to be inconsistent have not been correct; rather, it is sound.” (3) This is because Ibn al-Salah, may Allah have mercy on him, said that it was inconsistent, and an inconsistent hadith is one of the categories of weak hadith. A sound hadith is authoritative, because, according to the well-known definition, it establishes a preponderant probability: it is that which is narrated by an upright transmitter whose precision is somewhat less than complete, through a connected chain, and is free from irregularity and any discrediting defect.

Accordingly, the hadith is authoritative. Thus, if you cannot find an upright object