Is the saying attributed to Abu Bakr, may Allah be pleased with him: "By Allah, if one of my feet were in Paradise and the other outside it, I would not feel safe from Allah's مكْر (scheming/devising); for no one feels safe from Allah's مكْر except the disbelievers," authentic, knowing that the Messenger of Allah, may Allah's peace and blessings be upon him, gave him glad tidings of Paradise?

The noble verse: "Did they then feel secure against the plan of Allah? But no one feels secure from the plan of Allah except the losing people" warns against feeling secure from Allah's plan. As for the statement mentioned in the question, it has not been attributed to Al-Siddiq (may Allah be pleased with him) in any reliable source. It is well-known that the Companions who were given glad tidings of Paradise feared a bad ending despite the good news. This confirms that fear coexists with glad tidings and does not imply a lack of trust in Allah, due to the possibility of a condition that Allah did not reveal to them as a test.